X-Ray Measurements of Deformations in Films and Substrates in Heteroepitaxial SYSTEM GaAs/Ge
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INTRODUCTION The GaAs/Ge system is characterized by a very low mismatch : as the lattice parameters are very close (aGaAs = 5.6533 A , aGe = 5.6567 A ) the misfit f is notably low : 7.6 10-4 at room temperature. This allows for a high critical thickness in the epilayers, and progressive formation of misfit dislocations (M.D.) Moreover, very perfect epitaxial films can be obtained even when the theoretical critical thickness (as defined in Matthews model 1) is exceeded (metastability). X-Ray Topography allows one to obtain two kinds of information on epitaxial
monocrystalline systems , provided the layer is thick enough to give diffraction: -- radius of curvature can be correlated to the strain and thereby the stress in the sample. The elastic theory gives their ratio between film and substrate; -- tetragonal parameter in the layer gives the actual elastic deformation of the film; the measured value would be the same as the theoretical elastic one until relaxation occurs in the interface. First results were proposed in a previous paper in which nucleation mechanisms were reported and detailed 2. The aim of this paper is to show that XRay study of both curvature and tetragonal deformation is a powerful way to determine the whole mechanical state of an epitaxial system. First, it is shown that continuum isotropic elasticity successfully predicts the tetragonal lattice distortion of strained layers as well as the radius of curvature of specimens. Then final (elastic or relaxed) and initial states of the samples are determined. EXPERIMENTS
0.3 ýlm - 4 itm thick GaAs layers have been deposited on 200 grn - 260 grm (100) Ge substrates by Close-Spaced Vapor Transport (CSVT) 3 . This growth technique is based on the thermal dependence of the chemical reaction 4: 313
Mat. Res. Soc. Symp. Proc. Vol. 356 01995 Materials Research Society
2 GaAs (s) + H20 (g) t-* Ga 20 (g) + As 2 (or 1/2 As4 )(g) + H 2 (g) (1) The GaAs source and the Ge substrate are enclosed in a reactor and separated by a thin spacer (1 mm in general). They are maintained at different temperatures: 800'C for GaAs and 750'C for Ge, by graphite susceptors. At 800'C, H 20 reacts with GaAs according to the reaction (1). The reverse reaction occurs on Ge. The driving force for the deposition is the temperature gradient of 50'C which allows a net transport of material from the source to the substrate. Two topographic techniques were applied, giving complementary information : S\
stBerg-Barrett
setting
(ýX Lagetn
•
RX0
:j(,O.%
hkl)ýq
a
b
Figure 1: Diffracting planes used in Lang and Berg-Barret topography XR Transmission Topography (XRTT), using planes perpendicular to the interface (Lang setting, fig.la ; radiation Ag Kcz, X = 0.56 A). The scattering from the film is the same as the substrate and the images contain information about the whole sample. The (022) planes are oriented in the Bragg reflection position; as the sample is curved, they are no longer parallel from one end to the other (fig.2). So R can be deduced from the angular displacement of
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